The partitioning of arsenate between Paraho indirectly retorted and directly retorted oil shales and a combusted oil shale was examined with batch equilibrium adsorption isotherms. Arsenate adsorption was found statistically different affinities for arsenate. The greater adsorption capacity of combusted oil shale for arsenate was attributed to greater surface area and free iron oxide. Arsenate adsorption by combusted oil shale was not reversible. Upon dilution of the solution phase, arsenate did not desorb. Upon dilution of the retorted oil shale solutions, arsenate continued to be removed from solution. An evaluation of metal arsenate. stability in the spent oil shale systems indicated that the retorted oil Shale solutions were highly supersaturated with respect to magnesium and barium arsenates, whereas the combusted oil shale solutions were not supersaturated . The data were interpreted to indicate that adsorption reactions control arsenate solubility at short reaction times. As reaction times increase, precipitation reactions control soluble arsenate concentrations.